U.S. Pat. No. 2,606,925 describes the hydrogenation of aromatic compounds which are substituted directly on the aromatic ring via nitrogen atoms (for example aromatic amines or nitro compounds) to the corresponding cycloaliphatic compounds. The catalysts used are elemental ruthenium metal, ruthenium oxides, ruthenium salts, ruthenium sulfides and ruthenium sulfates. Optimal results can be achieved according to this document when the ruthenium is present in finely distributed form. However, no method of preparing these catalysts is described.
In U.S. Pat. No. 2,494,563, finely divided Ru catalysts are used for the hydrogenation of aromatic diamines to the corresponding aliphatic diamines.
A method for preparing very small ruthenium dioxide particles by reacting RuCl3 solution with NaOH in the aqueous phase, washing and drying is described in CA-A-860855. This affords crystal sizes less than 50 nm. A preferred use specified for the powder obtained was the production of electrical resistors.
DE-A-2132547 discloses a process for hydrogenating aromatic compounds to the corresponding cycloaliphatic. For the hydrogenation, a catalyst based on oxide hydrates of Ru is used. The catalyst is prepared by precipitation from an aqueous solution of a ruthenium salt by addition of alkali metal hydroxide solution. The ruthenium oxide hydrate thus obtained can be used directly in the process or be subjected to drying before use. After the drying, the catalyst, according to the disclosure, is present in the form of powder with particle sizes in the range from 4 to 6 nm, the Ru being present in the resulting dry powder as Ru(IV) oxide hydrate with approx. 50% by weight of Ru.
DE 101 19 135 A1 describes the continuous preparation of bis(4-aminocyclohexyl)methane by hydrogenation of bis(4-aminophenyl)methane with the aid of a pulverulent ruthenium catalyst. The ruthenium was used applied to an inert support material, e.g. Al2O3, and with a particle size of 5-150 μm.
U.S. Pat. No. 5,214,212 teaches the addition of metal salts as promoters in a process for hydrogenating aromatic amines. According to the disclosure, the addition of promoters leads to an improvement in the reaction rate and to a reduction in by-product formation. The metal salts were used in concentrations of from 0.3% to 10% based on the aromatic amine.
U.S. Pat. No. 3,864,361 describes the preparation of 2,5-dimethylpyrrolidone by reduction of 2,5-dimethylpyrrole in the presence of finely divided, unsupported RuO2. After the hydrogenation has ended, the catalyst is removed by filtration. According to the disclosure, the removal of the Ru catalyst can be improved by the addition of Al2O3 as a filtering aid.
The prior art teaches the use of heterogeneous Ru catalysts with small particle sizes, which should be present in very fine distribution in the reaction mixture. When such catalysts are used in an industrial scale process, however, it is found that the small catalyst particles form relatively large agglomerates during the hydrogenation, which can then settle out in the reactor. This firstly removes a portion of the amount of catalyst from the process; secondly, the resulting deposits can cause blockages which lead to disruption in the course of operation.
In the aromatic starting materials which are used in the process for preparing cycloaliphatic amines, there are typically small amounts of by-products which generally cannot be removed quantitatively during the purification of the starting materials. These by-products can poison the ruthenium catalyst and hence reduce the space-time yield of the process.
In the case of the hydrogenation of bis(4-aminophenyl)methane or bis(4-amino-3-methylphenyl)methane to the particular cyclohexyl derivatives, these by-products are principally the hydrochlorides of the aromatic starting amines and higher-boiling aromatic products.
Even when the chlorine content of the particular starting materials is higher than 1 ppm or the concentration of higher-boiling products exceeds the value of 2% by weight, considerable losses in the space-time yield can arise.